The present invention relates to nanostructures and nanostructure formation techniques, and more particularly, but not exclusively, relates to metallic nanowires.
With the constantly decreasing feature sizes of integrated circuits and other microscale devices, the need for increasingly fine patterning of such features present several challenges. Among these challenges is the need to provide various nanostructures, including electrically conductive nanowires. One of the schemes to fabricate nanowires utilizes a lithographic pattern to define an array of closely spaced nanowires that are deposited by chemical vapor deposition or another technique. One drawback of this approach is that lithographic patterning is limited in terms of size and tolerance in the nanometer range.
A related scheme involves filling arrays of channels or pores in a substrate with a material of interest by chemical or electrochemical processes. With this method, it is difficult to continuously fill pores having a relatively long length and small diameter with a desired material having high density. In some instances, to adequately fill the pores, high pressure and high temperature injection of molten metals has been used; however, the fabrication of nanowires from small pores with long length frequently becomes undesirable because of the extreme pressure/temperature involved. Attempts to lower the processing pressure by modifying the composition or surface property of the substrate pores have the further limitation of being applicable with only low melting point metals.
Another scheme bombards a material with ions to form tracks into which the nanowire material is deposited. The track-bearing material is then at least partially removed, leaving the exposed nanowires. U.S. Pat. No. 6,444,256 B1 to Musket et al. provides further background information regarding this approach. Such schemes are complicated by the need to remove the material that defined the tracks. A similar drawback arises for other approaches that utilize a template to define channels or pores for forming nanostructures.
An article by Ross et al. describes the growth of copper columns by ion-induced chemical vapor deposition (IT-CVD) in which a carbonaceous material is positioned between the copper columns. This material extends from the boundary of one column to the next, and separates the columns to provide a solid structure about them. Ross et al., Ion-Induced Chemical Vapor Deposition of Copper Films with Nanocellular Microstructures, APPLIED PHYSIC LETTERS, vol. 83, No. 6 (11 Aug. 2003).
A step-edge decoration method has been reported to grow freestanding nanowires, where Mo oxide wires were electrodeposited at step edges on a graphite surface and reduced in H2 to produce metal. This method is not applicable to produce noble metals, including gold (Au), silver (Ag), platinum (Pt), and copper (Cu), which typically do not nucleate along step edges on graphite to form nanowires. Another method reported is to synthesize Au, Ag and Pt nanowires using reducing and sacrificing templates. Neither step-edge decoration nor sacrificing template methods are capable of producing vertically aligned freestanding metal nanowires. Still other schemes involve a catalyst to provide for the freestanding deposition of carbon nanotubes to which a metallic coating may then be applied. U.S. Pat. No. 6,340,822 B1 to Brown et al. provides further background regarding this approach. These schemes all fail to provide for the growth of freestanding nanowires comprised of metals without complicated post-growth processing. Thus, there is a demand for further contributions in this area of technology.